III-V nitride resonate based photoacoustic sensor

ABSTRACT

The invention relates to a micro cantilever beam sensor and making method, including a chip, and its character: there is at least a group of sensor cells set on the chip, where the sensor cell is composed of the completely same four force-sensitive resistors composing a Wheatstone bridge and two cantilever beams, two of these resistors are on the substrate of the chip, the other two are on the two cantilever beams, respectively, one cantilever beam acts on a measuring cantilever beam and the other one acts on a reference cantilever beam, and the measuring cantilever beam is set with a sensitive layer on the surface. It can design and prepare in a liquid-flow micro-tank by front etching and silicon-glass bonding techniques, to directly detect liquid biomolecule. Whether applied to gas sensor or biosensor, it will play an important role in reducing device size, enhancing device sensitivity and realizing sensor multi-functionality. It has wide prospects for the fields of environment monitoring, clinic diagnosis and therapy, new drug development, food safety, industrial processing control, military and so on.

PRIORITY INFORMATION

The present application is a divisional application of U.S. patentapplication Ser. No. 14/824,269, filed on Aug. 12, 2015, which claimspriority to U.S. Provisional Patent Application Ser. No. 62/070,038titled “III-V Nitride Microcantilever Based Photoacoustic Biosensor” ofKoley, et al. filed on Aug. 13, 2014, the disclosure of which isincorporated by reference herein.

GOVERNMENT SUPPORT CLAUSE

This invention was made with government support under ECCS-0801435awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

Development of ultrasensitive micro- and nano-electromechanical systems(MEMS/NEMS) has resulted in ultra-high detection sensitivity, offeringsub nanometer scale displacement detection, zeptogram level masssensing, single bio-molecular sensing, and atomic resolution imaging.Micro and nanocantilevers, as MEMS/NEMS transducers, have been usedextensively for these sensing applications. Optical transduction ofcantilever motion is almost exclusively used to achieve high deflectionsensitivity (in the femtometer range), but it suffers from high powerrequirement, challenges with miniaturization and array based operation.Femtometer scale displacement detection using nanocantilevers operatingat several hundred MHz has been demonstrated, but is limited by itschallenging fabrication and integration schemes, coupled withcomplicacies of impedance matching for high frequency signaltransmission. Silicon (Si) based piezoresistive microcantilevers havebeen developed which are easily integrated for array based operation,but have low sensitivity offering displacement resolution in the rangeof nanometers.

Instead of a simple piezoresistor, embedding a transistor at the base ofthe microcantilever (henceforth to be called a “piezotransistive”microcantilever) to transduce its deflection is an attractive way todramatically improve its sensitivity by orders of magnitude, since thegate can be utilized to control the charge carrier density and themobility of the carriers in the channel.

Recently, metal oxide semiconductor field effect transistor (MOSFET)integrated Si cantilevers have been proposed with the goal of achievingvery high deflection sensitivity while avoiding the challengesassociated with the aforementioned techniques. Although thesemicrocantilevers showed high sensitivity in the nm range for stepdeflections, since its high sensitivity supposedly originated fromtrapping effects in the MOSFET, it is difficult to reproduce thesesensors, or operate them at high frequencies. Indeed, Si basedpiezotransistive microcantilevers are theoretically incapable ofexhibiting direct sensitivity enhancement through gate control, sincethe piezoresistive effects in Si originate from the variation in carriermobility due to strain related splitting of the conduction band minimaenergy levels. On the other hand, piezotransistive cantilevers made ofpiezoelectric materials can directly utilize the charge densityvariation caused by the deflection induced strain to exhibit highsensitivity with very high repeatability.

Due to strong piezoelectric properties of AlN and GaN, AlGaN/GaNheterojunction, provides a unique avenue to translate the staticpiezoelectric charge generated at the interface due to applied straininto a change in resistance of the two dimensional electron gas (2DEG)formed at the interface, since the generated piezoelectric charge canproportionately modulate the density of the 2DEG. In addition tochanging the carrier density, the applied strain can also change thecarrier mobility by changing their effective mass. The utility ofAlGaN/GaN heterojunction based piezoresistor (for step bending anddynamic deflection measurements) and piezotransistor (for staticdeflection measurements) has been demonstrated, however, the effect ofgate in enhancing displacement sensitivity down to femtometer range inhigh frequency dynamic deflection mode, with subsequent applications inunique analyte detection, has never been realized.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

A microcantilever based photoacoustic sensor is generally provided. Inone embodiment, the microcantilever includes: a substrate; a galliumnitride (GaN) layer on the substrate, wherein the GaN layer defines acantilever extending beyond an edge of the substrate, with a base areaof the cantilever defined by the area spanning the edge of thesubstrate; a heterojunction field effect transistor (HFET) deflectiontransducer positioned on the cantilever; a pair of electrical contacts,each electrically connected to the HFET deflection transducer; and amicrofluidic channel in fluid communication with an analyte reservoir,wherein the analyte reservoir is positioned at the base of thecantilever.

A sensing system is also generally provided. In one embodiment, thesensing system includes: the microcantilever based photoacoustic sensorof the preceding paragraph and a pulsed infrared laser positioned todirect infrared electromagnetic radiation into the reservoir of themicrocantilever based photoacoustic sensor.

A method is also generally provided for detecting an analyte within amedium. In one embodiment, the method includes: flowing the mediumwithout any analyte present through the microfluidic channel of thesystem of the preceding paragraph with the pulsed infrared laserdirecting infrared electromagnetic radiation into the reservoir toestablish a baseline signal created from baseline cantileveroscillations; flowing an unknown sample containing the medium throughthe microfluidic channel of the system with the pulsed infrared laserdirecting infrared electromagnetic radiation into the reservoir toestablish a baseline signal; wherein the presence of the analyte in theunknown medium changes the baseline signal.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures.

FIGS. 1a-1c sequentially show an exemplary GaN microcantilever beingfabricated, with:

FIG. 1a showing microfludic lines being etched and coated with apoly(p-xylylene) polymer (e.g., PARYLENE®);

FIG. 1b showing a thin sapphire rectangular film glued on top of thechannel for vacuum encapsulation; and

FIG. 1c showing polymer tubing (e.g., TYGON®) lines epoxy glued topolydimethylsiloxane (PDMS) block to connect to the microfluidicchannel.

FIG. 2a shows a schematic diagram of an exemplary measurement set up fordetection of an analyte in the flowing sample. In this embodiment, thethin sapphire rectangular piece seals the vacuum from the top side, andthe bottom of the chip seals to the chip carrier to maintain vacuumaround the microcantilever resonator. Signals from the Au contacts willbe taken out to a lock-in amplifier.

FIG. 2b shows an exemplary system including the microcantilever basedphotoacoustic sensor, a laser (e.g., pulsed infrared laser positioned todirect infrared electromagnetic radiation into the reservoir of themicrocantilever based photoacoustic sensor), an amplifier connected tothe sensor, and a general purpose computer connected to the amplifier.

FIG. 3 shows a cross-section of the different layers of the AlGaN/GaNwafer grown on Si (111) substrate, with mesa and cantilever layer asshown, according to Example 1.

DETAILED DESCRIPTION

Reference now will be made to the embodiments of the invention, one ormore examples of which are set forth below. Each example is provided byway of an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly, and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

In the present disclosure, when a layer is being described as “on” or“over” another layer or substrate, it is to be understood that thelayers can either be directly contacting each other or have anotherlayer or feature between the layers, unless expressly stated to thecontrary. Thus, these terms are simply describing the relative positionof the layers to each other and do not necessarily mean “on top of”since the relative position above or below depends upon the orientationof the device to the viewer.

As used herein, the terms “first”, “second”, and “third” may be usedinterchangeably to distinguish one component from another and are notintended to signify location or importance of the individual components.

Chemical elements are discussed in the present disclosure using theircommon chemical abbreviation, such as commonly found on a periodic tableof elements. For example, hydrogen is represented by its common chemicalabbreviation H; helium is represented by its common chemicalabbreviation He; and so forth.

Method of fabricating GaN microcantilevers with a AlGaN/GaNheterojunction field-effect transistor (HFET) integrated at the basewith very high gauge factor (e.g., greater than about 4500) is provided,along the resulting GaN microcantilevers. Thus, instead of a simplepiezoresistor, the transistor imbedded at the base of themicrocantilever (henceforth to be called a “piezotransistive”microcantilever) can transduce its deflection is an attractive way todramatically improve its sensitivity by orders of magnitude, since thegate can be utilized to control the charge carrier density and themobility of the carriers in the channel. Additionally, fully electronicreadout of the deflection of such a microcantilever in both dc and acconditions is generally provided. Ultrasonic sensing based on III-VNitride Microcantilever in air and in solid medium is also demonstratedherein using piezoresistive (all electronic) cantilevers. Vacuumenclosed cantilevers are also provided for photoacoustic sensing, suchas for photoacoustic detection of biological cells, various cancercells, especially melanoma cells. Additionally, cell, protein molecules,and DNA identification methods are provided herein based onphotoacoustic spectroscopy, along with detection of tumors throughphotoacoustic microscopy and imaging.

In one embodiment, the present disclosure is generally directed to ahighly sensitive and miniaturized sensor, which in certain embodimentsis capable of detecting a single analyte in a circulating liquidsamples. The sensor utilizes photoacoustic detection techniques todifferentiate a targeted analyte from other analytes present in samplein a label-free manner, while system miniaturization can be achieved byusing novel and highly sensitive gallium nitride based piezoelectricmicrocantilever (resembling tiny diving boards) sensors describedherein.

Thus, the usage of a highly sensitive microscale and novel III-V Nitridebased piezoelectric sensor is generally provided in one embodiment toperform photoacoustic spectroscopy. Also, the usage of the microscalesensor is generally provided to develop an integrated label-freedetection platform that can be miniaturized to less than about 1 cm indimensions. Additionally, the utilization of the concept of enclosingthe microcantilever in a high vacuum chamber is generally provided, inone embodiment, to obtain 2-3 orders of magnitude higher sensitivitycompared to air operation. Finally, the integration of semiconductormaterials with appropriate properties is generally provided, such asIII-V Nitrides, which are transparent to probe IR radiation, are used tofabricate the microcantilever sensor (to reduce noise), while Sisubstrate is used, which offers a mature process technology and usefulfor future integration of devices on the same chip. The possibility ofintegrating the IR laser (diode laser chip) in the sensor throughheterogeneous integration is also generally provided, since they can bestacked to just increase the thickness of the chip, and not its lateraldimensions which will make it a highly compact photoacousticspectroscopic detection system and can also have a large array ofsensors for simultaneous screening.

A microcantilever based photoacoustic sensor (operating in atmosphericpressure) has also been shown to be 2-3 orders of magnitude higher thana commercially available large area piezoelectric sensor.

In the system disclosed herein, and referring to FIGS. 1a-1c and FIGS.2a-2b , the photoacoustic sensor 100 is able to uniquely identifymolecules if spectroscopy is performed, i.e. vary the wavelength of theincident IR laser 126. Additionally, the microcantilever 101 basedphotoacoustic sensor 100 can be designed to maximize the detectionsensitivity with the ultimate goal of detecting even extremely faintacoustic signal or wave 122 arising from a single cell. In the device,pressure waves are detected when they are created by the melanoma cells120 in blood as they absorb pulsed IR radiation 130. Thus, more complexsimulations and theoretical modeling combining thermal, acoustic,mechanical, and electrical effects will be performed to determine thebest design for our sensor. The major sensor parameters to be optimizedare the dimensions of the microcantilever 101, the position anddimension of the reservoir 106 for holding analytes. The former woulddetermine two important mechanical parameters, resonance frequency andquality factor, while the latter would determine the signal intensityreaching the microcantilever 101. For the former the targeted valuewould be about 50 KHz to make sure that fast measurements can be made,while for the latter, a value of greater than about 50,000 in highvacuum is preferred, which would result in high detection sensitivity.

A. Microcantilevers

Microfabricated cantilevers have been used in atomic force microscopy(AFM) for more than 20 years. Cantilevers (like a tiny diving board orbeams with one fixed and one free end) have been widely used in recentyears as miniaturized, ultrasensitive, and fast-responding sensors forapplications in chemistry, physics, biochemistry, and medicine.Microcantilever sensors such as the sensor 100 shown in FIGS. 1a-1c andFIGS. 2a-2b respond not only by bending (static mode) due to theabsorption of molecules, change in pressure, temperature, andelectrostatic field; as well as shift in resonance frequency, change inamplitude of oscillation 124 (see FIG. 2b ) also occurs in dynamic mode.Over a decade, microcantilever based sensing has witnessed an impressiveprogress due to multi-disciplinary scientific research, evident from thenumber of publications in the last 10 years. In the last decade,microcantilever based sensors have proved to become a versatiletransduction platform for physical parameters, chemical, volatileorganic molecules, explosives and biomolecule detection.

Silicon (Si), the most abundant and matured technology, has been alwaysconsidered as the prime material in semiconductor industries everywhere.However, application of Si has shown limitations in sensing applicationsin harsh environmental conditions, suffering from low sensitivity andselectivity. Si cannot be used for high temperature applications as itloses the electrical and mechanical reliability at about 500° C. One ofthe great advantages of the wide band gap semiconductors is their veryhigh mechanical, thermal, chemical and biochemical stability, whichoffers exciting MEMS/NEMS sensing applications which requirereliability, linearity, sensitivity, and selectivity. Moreover,materials with a high Young's modulus can better maintain linearitybetween applied load and the induced deformation. This particularlydemands group-III nitrides, which has high Young's modulus. AlGaN/GaNheterostructures contain a highly conductive two-dimensional electrongas (2DEG) at the interface, which is sensitive to mechanical load, aswell as to chemical modification of the surface, and can be used fornovel sensing principles. Presence of such a 2DEG is unique to AlGaN/GaNheterostructure, and is attributed to unintentional polarization doping,since it arises because of the strong polarization properties of thenitrides. Among the most common semiconductors AlN (6.13 eV) and GaN(3.42 eV) have much higher bandgap compared to others. Due to such widebandgaps, their critical electric fields for breakdown are much higherthan other III-V semiconductors. Though GaAs has much higher low fieldmobility, GaN is clearly superior in terms of saturation velocity.Together with high bandgap, high saturation velocity and high mobilitymakes nitrides ideal contenders for high power microwave application.The presence of a direct and wide bandgap in AlGaN/GaN also make themvery suitable for optoelectronic applications, especially in the green,blue, and UV regions of the spectrum, where there are virtually no othercontenders.

The intensity of the acoustic pressure waves reaching the cantileverdepends on the position of the reservoir, where they are created by thelaser spot. The laser wavelength and power (typically ˜800 nm, and 100mW, respectively) producing the best detection sensitivity of themelanoma cells over the background (red blood cells, white blood cellsetc.) can be determined. The magnitude of the electrically obtainedsignal, i.e. electrical sensitivity, is a strong function of the channelresistance which can be tuned with the gate bias of the field-effecttransistor (FET). Appropriate gate bias can be optimized (e.g., viamodeling and comparison to experimental data) to obtain the bestdeflection transduction (and sensitivity) possible for the sensor.

Referring to FIGS. 1a-1c , FIGS. 2a-2b , and FIG. 3, the development ofthe sensor 100 involves the fabrication of a III-V Nitride basedmicrocantilevers 101 with embedded HFETs 108, integration of themicrofluidic channels 102 for analyte transport, sealing the sensor inhigh vacuum, and finally wire bonding to a chip carrier. Themicrocantilevers 101 can be fabricated through a process that involvesseparate lithographic steps to define the AlGaN mesa 128, followed bycantilever 101 outline definition, deposition of source/drain and gatemetal contacts 104, and finally, through wafer backside Si etching usingBosch process to release the cantilever 101. The microfluidic channel(40×40 μm cross-section) 102 and reservoir 106 near the base of thecantilever 101 can be patterned by introducing a separate fabricationstep in the process, in which the GaN layer 118 and the Si or substratelayer 112 are etched to appropriate depth and width. The analytereservoir 106 can have dimensions suitable such that the cells can beadequately contained within it for the incident laser spot to hit them(e.g., 60×60 μm).

The reservoir walls are, in one embodiment, coated with apoly(p-xylylene) polymer such as PARYLENE® (e.g., at a thickness ofabout 1 μm) to ensure better acoustic impedance match between theanalyte liquid and Si (so that ultrasonic waves can propagate betweenthe two media with low attenuation) as well as to ensurebio-compatibility.

The major fabrication steps for the sensor 100 are shown in FIGS. 1a,1b, and 1c , with: FIG. 1a showing microfludic lines or channels 102being etched and coated with a poly(p-xylylene) polymer (e.g.,PARYLENE®) onto a microcantilever device 101 (e.g., as shown in FIG. 1c); FIG. 1b showing a thin sapphire rectangular film 110 glued on top ofthe channel for vacuum encapsulation; and FIG. 1c showing polymer tubing(e.g. TYGON®) lines 114 epoxy glued to polydimethylsiloxane (PDMS) block116 to connect to the microfluidic lines or channels 102.

In one embodiment, the microcantilever has a length of about 50 μm toabout 300 μm, a width of about 25 μm to about 100 μm, and a thickness ofabout 0.5 μm to about 1.5 μm.

Ultra low vacuum encapsulation is a challenge for MEMS devices. However,vacuum operation of the device is initially achieved by sealing the toplayer with an IR transparent sapphire window (e.g., having a thicknessof about 100 μm), and attaching a pump outlet to a small orifice in thechip to produce the necessary vacuum. High vacuum (low milliTorr or evenμTorr levels) sealing methods can be used to obtain standalone sensorchips with advanced packaging methods such as, solder bonding or evenwafer bonding at the bottom surface of the chip (following the initialtop layer sealing). Following device fabrication, and vacuum sealing,wire bonding can be done to the leads of the chip carrier. Finally, theexternal microfluidic lines (to flow the analytes) can be attached tothe microfabricated channels using PDMS molding. The sensor can beconnected to data acquisition systems (not shown) for testing andmeasurement.

Melanoma Cancer Cell Detection:

To be clinically relevant, the sensor can detect 1 tumor cell in abackground of 106-107 normal blood cells. To simulate circulatingmelanoma cells in the blood stream, melanoma cells can be suspended indefibrinated bovine blood with a concentration of 1-103 cells/ml. Theblood flow can be generated by a syringe pump, and small polymer tubes(e.g., TYGON®) can be epoxy glued to the PDMS molds at the end of thePARYLENE® coated microfluidic channels to circulate the test fluid. Thevolume flow rate of blood can be set to correspond to a blood flow rateof several cm/s in the microfluidic channels, similar to that in humanarteries.

Referring to FIGS. 2a and 2b , for detecting the melanoma cells, a 800nm incident IR laser 126 (the wavelength can be changed as needed toobtain the best resolution) with a power of 100 mW (which can also bechanged as needed to obtain the best resolution), can be pulsed on thereservoir 106 at the base of the cantilever 101 at its resonantfrequency. The sensor 100's signal can be measured using a lock-inamplifier 134 and recorded by a computer 136 as the melanoma cells 120absorb pulsed IR radiation 130. The baseline signal can be recorded inpresence of normal cells, which increases significantly in presence ofmelanoma cells, depending on their size and melanin content, as melanomahas two orders of magnitude higher IR absorption coefficient compared tonormal blood cells due to presence of melanin. Thus, detection of thepassage of individual melanoma cells through the channel is possible, asis detection of the variation in their melanoma content, in real time(on the order of a few ms), which is currently not possible usingexisting techniques.

Example 1

Fabrications of AlGaN/GaN HFET/MOSHFET (metal-oxide-semiconductorheterojunction field-effect transistor)/MISHFET(metal-insulator-semiconductor heterojunction field-effect transistor)are well documented. Here, for the first time, the complete fabricationdetails, issues, and solutions are disclosed of several novel AlGaN/GaNHFET/MOSHFET/MISHFET embedded GaN microcantilevers. Although theprinciples and application of different devices vary from each other,but the fabrication processes remain the same.

Wafer Information

Referring to FIG. 3, a six inch AlGaN/GaN wafer grown on Silicon (111)substrate 112 was purchased from NTT ADVANCED TECHNOLOGY CORPORATION,Japan for this work. The wafer was diced into ˜44 (1.8 cm by 1.8 cm)square pieces. Before dicing, the wafer was spin coated with photoresist (SHIPLEY 1827) and then baked for 5 mins at 110° C., solely toprotect the top surface from any damage may happen during wafer dicing.Silicon substrate 112 (111) of ˜720-800 μm thickness was used to growthe AlGaN/GaN layer. A 300 nm buffer layer 140 was used as a transitionlayer before growing 1 μm undoped GaN layer 119, although a buffer layerof from about 0.5 μm to about 2.5 μm can be used and an undoped GaNlayer of from about 500 nm to about 2 μm can be used. Together, thistransition layer and the undoped GaN layer form the thickness of themicrocantilevers 101. On the top of the GaN layer 119, a thin layer of 1nm AlN 138 was used to form abrupt junction and better electronconfinement in 2DEG by tuning the bandgap. Above that layer, the activelayer of AlGaN 128 of 15 nm and 2 nm of GaN cap layer 118 arepositioned.

Two 5″×5″×0.09″ masks (material:chrome, substrate:quartz) were orderedfrom PHOTO SCIENCES INC., USA after designing in AutoCAD 2013. Therewere 7 lithographic layers in the fabrication process (described indetails in the next section), all the layers were designed and threecopies of each layer were organized in two masks. Three layers (Mesaisolation, GaN cantilever outline, and Backside Si etch) were 1.8 cm by1.8 cm box equal size of the sample and other three layers were 1.4 cmby 1.4 cm. These layers could be made exact size as others, that makesthe alignment task easier but it will consume more space in the mask. Ifthere is plenty space in the mask, it is better to have equal sizedlayers and also equal to the sample size. The mask was clear field. Theback side alignment layer for through wafer Si etching should bemirrored respect to the first two top layers if the design hasasymmetry. If it is a symmetric design then mirroring the back sidelayer would not be necessary. The wafer was diced 1.8 cm by 1.8 cm,though all the devices would fit 1.4 cm sample size. The only reason tohave some empty space around the sample for handling with tweezers. Alsolater in this section, readers will find why it is useful to keep morespace around the actual device area. The first two layers specially GaNoutline layer and the back side layer should have a ‘+ sign’ for autodicing each sample into small pieces as it will be really hard to dicethe small samples further after final release of cantilevers. Whiledesigning the mask, it is easy to start from the GaN outline. Afterdrawing the complete device, then separate each layer and organizeaccording to the size of the mask. PHOTO SCIENCES has its own rulesabout drawing, and they have to be followed for faster processing. Whenthe mask is made, the mask should be thoroughly checked for any damage,design violation, and sharpness of chrome line.

Microcantilever Design

Positive photo resist (PPR, SC 1827) was used for the first processstep, whereas negative photo resist (NPR, NR 71) was used for the restand NR 5 was used in Bosch process for releasing cantilevers.

Step 1—MESA Outline:

Mesa is the active region on which the AlGaN/GaN HFET is fabricatedbecause the AlGaN/GaN layer has 2DEG throughout the wafer and thereforeis conductive all over and needs to be isolated from other patterns onthe sample. Only in this layer PPR SC 1827 was used. Plasma-enhancedchemical vapor deposition (PECVD) SiO2 (300-400 nm) was deposited usingUNAXIS PECVD tool (deposition rate is 50 nm/min) at the beginning. Theoxide was patterned and then etched in Plasma Therm Inductively CoupledPlasma (ICP) tool (etch rate is 180 nm/min, CHF₃/O₂ gas). Then usedBCl₃/Cl₂ based dry etching recipe of GaN in inductively coupled plasma(ICP) to etch 180-200 nm to isolate mesa. Though more than 15 nm ofAlGaN etching would be sufficient but over etch is done to ensurecomplete isolation and also for next alignment purpose (below 100 nmthickness would be harder to see in MA6). After the etching, thephotoresist (PR) should be completely removed from top oxide layerfollowing resist remover, oxygen plasma cleaning in Reactive Ion Etcher(ME), and if necessary dipping in warm sulphuric acid (H₂SO₄) for 5-10minutes. The resist gets crosslinked in ICP, and it becomes literallyimpossible to remove with just resist remover or acetone. That is why itis better to have the oxide layer protecting the mesa which acts as thehard mask. Otherwise without oxide deposition, mesa etching can still beperformed. It is suggested that after mesa etching, the sample should bekept in warm resist remover (MICROPOSIT 1165) for 10-30 minutes and thencleaning the sample with cleanroom swab (soaked in the same remover tomake it soft and not to scratch the sample). If this cleaning is notsufficient then oxygen plasma etching would be needed. Keep in mindthat, bare AlGaN/GaN mesa should never be exposed in oxygen plasma,otherwise 2DEG would be completely damaged.

Step 2—GaN Cantilever Outline:

In this step, GaN is etched down to make an outline for the cantilever.GaN is etched down in the pocket area up to the substrate where silicongets exposed. This process was exactly same as step 1. Only differenceis the deposited oxide is 1.2 μm thick as the remaining thickness of GaNafter etching for mesa in step 1 is about 1.1 Over etching (assuming 2μm thick GaN) is performed as the etched down GaN has other layers.BCL₃/Cl₂ also etches exposed Si (verified using Tencor Profilometer)with same etch rate of 340 nm/min, but this does affect any fabricationprocess as ultimately the exposed Si will be etched from backcompletely. In this step and the next ones in this sub-section, negativephoto resist (NPR) NR 71 was used. After the etching of oxide similarlyas step 1, resist should be removed. After resist removal, wet chemicaletching of the oxide is done using Buffered Oxide Etchant (BOE).

Step 3—Ohmic Contact:

For ohmic contact multilayer gate metal stack of Ti (20 nm)/Al (100nm)/Ti (45 nm)/Au (55 nm) was used. Getting a good ohmic has always beena challenge and multilayer metal stack gives low contact resistance. Fora good and easy metal liftoff process, overdevelopment is suggestedafter post bake of resist as very thin layer of resist would be alwayspresent. Also, the extra space surrounding the sample should be used tomount the sample with polyamide film (e.g., KAPTON® tape) in CVCElectron Beam Evaporator's holder. So that the metal does not getdeposited on the edges which makes the liftoff very hard and timeconsuming. The metal liftoff should be done in warm resist remover(RR41), submerging the sample for as long as the unnecessary metal filmcomes off. After that, the sample was put in fresh warm resist removerfor 10-15 minutes and the using soaked (in RR41) cleanroom swab is usedto clean the sample by whirling the swab. When satisfied (checking inmicroscope to ensure no resist is left), the sample should be cleanedwith squirting IPA after every successive whirling with swab soaked inresist remover. No oxygen plasma cleaning should be done on the samplewith bare AlGaN/GaN mesa. However as the GaN outline has already createdseveral trenches in the sample, resist becomes highly adhesive to thesurface, and so warm H₂SO₄ treatment can be performed. Every after 1-2minutes, the sample should be checked to ensure no unwanted liftoff ofohmic contacts is happening. It happens because of thin layer of resiststill present underneath the metal contacts. After lift-off is done, thecontact is annealed in rapid thermal processing (RTP) at 825° C.

Step 4—Schottky Gate Formation:

If the aim is to design simple piezoresistor then this step should beskipped. In this Example, the designed microcantilevers have manyvarieties in the FET part, where the samples are processed with orwithout gate dielectric. Liftoff process was followed to reduce theprocessing time and one lithography step which involves depositingdielectric materials and then pattern the gate layer to etch awaydielectrics from other areas on the sample. However, liftoff processeliminates that need and after patterning the sample with resist, gatedielectric can be deposited followed by gate metal and finally lift offthe resist as described in previous step. To create high Schottkybarrier with nitride surface, higher work function Schottky contacts areneeded and both Pt and Ni are ideal choices for Schottky gate contact.Ni is a preferred choice due to its higher adhesion property withnitrides and can be operable up to 600° C. Therefore a Ni/Au metal stackfor Schottky contacts for the HFET gates.

Devices were fabricated without dielectric (HFET), with Plasma EnhancedChemical Vapor Deposition (PECVD) of SiO2 (MOSHEFT structure), PulsedLaser Deposition (PLD) of Boron Nitride (BN), and Atomic LayerDeposition (ALD) of Al₂O₃(MISHEFT structure). PECVD oxide is the mostlyused gate dielectric which reduces the leakage by several orders as HFEThas high gate leakage. The film thickness was 5 nm. For PECVD, 5 nm ofoxide was deposited at 100° C. The usual recipe and the UNAXIS PECVDtool would not allow deposition below 250° C. but at that temperaturethe resist will burn and contaminate the chamber which is notpermissible. So if not possible to use the recipe with lowertemperature, one has to follow the etching of oxide film with an addedlitho step. In case of atomic layer deposition (ALD), 5 nm of Al₂O₃ wasdeposited with thermal oxide recipe at 100° C. The deposition rate is 1Å/cycle which takes more than an hour to deposit 5 nm film. This longerduration hard bakes the resist and eventually impossible to lift offespecially with smaller feature size. The only option would be to havethe film deposited first and the follow the etching procedure. Althoughdevices with oxide and BN were fabricated, the Al₂O₃ deposited deviceswere not continued for further processing due to limitation of time.

Step 5—Probe Contact:

Large metal pads (250 μm by 250 μm) are deposited for characterizationwhich connects to the drain, source, gate and cantilever tip. Gold withadhesion layer of Ti was used for this metal deposition step. The masklayout has two probe layers with long contact and short contact. Longcontacts are helpful for microfluidic channel integration, vacuumsealing of the sample, and utilizing fabricated micro-canals/discs whichare patterned in step 2. The lift off process remains the same asmentioned in step 3.

Through wafer Si etch from backside using Bosch process.

The cantilever is released by through wafer etching of Si using STS®inductively coupled plasma (ICP) etcher. We used ‘Bosch process’ wherethe etcher alternates between an ‘etch’ cycle and ‘passivation’ cycle.During the etch cycle, Si is isotropically etched using SF6 for 10seconds, then the etched region is passivated with a polymer (C4F8) for7 seconds in the passivation cycle. The whole process continuesalternatively as long as the cantilever is not released, resulting in ahigh aspect ratio Si etch with vertical side walls.

Through Wafer Si Etch from Backside Using Bosch Process

The cantilever is released by through wafer etching of Si using STS®inductively coupled plasma (ICP) etcher, using a ‘Bosch process’ wherethe etcher alternates between an ‘etch’ cycle and ‘passivation’ cycle.During the etch cycle, Si is isotropically etched using SF6 for 10seconds, then the etched region is passivated with a polymer (C₄F₈) for7 seconds in the passivation cycle. The whole process continuesalternatively as long as the cantilever is not released, resulting in ahigh aspect ratio Si etch with vertical side walls.

However, the usual practice of processing this particular layer involvesdepositing thick SiO₂ on the back side which acts as the hard mask forSi etching. Then patterning with NR 71 resist (4 μm thick), the oxide iswet chemically etched using buffered oxide etchant (BOE). The resist isthen removed from the backside and also from the top side (which acts asa protecting layer of the devices on the top side from spinner andbuffered oxide etchant (BOE). After that the sample is put into ICP toetch Si for releasing the cantilevers. This process is faster andeasier, however there are several key factors that affect the finaloutcome. In ICP the selectivity is about 90:1 between Si and SiO₂. For awafer of 500 μm thick (our first generation wafer from NITRONEX INC),the oxide needs to be 7-8 μm thick on the backside of the sample andalso in the carrier wafer. The carrier wafer is needed for mountingsmall samples with cool grease before loading in the inductively coupledplasma (ICP) chamber. Now if the pocket (where the Si will be etched) isbig enough and the layer has symmetric design with moderately thick Sisubstrate the above mentioned process works fine but will have lot ofundesirable undercut of Si, resulting in over hanged cantilevers. As themaximum strain is supposed to be at the base and the cantilever shouldbe the only suspended part, this process yields less sensitive devicesand in some cases devices of no use. This process becomes totallyinapplicable and impractical, in most embodiments, if:

(a) The thickness of Si wafer is above 600 as the thickness of oxidewould be more than 8 μm which would require longer tool time. Like ourrecent wafer which is 720-800 the oxide thickness should be more than 10The PECVD tool allowed 3 μm thick film deposition at a time, but thequality becomes bad. So it is advised to deposit 2 μm thick oxide (50nm/min deposition rate needs 40 minutes plus purging time yields aboutan hour), then run clean process for 2 hours and deposit again. Thatmeans more than 14 hours of total processing time is required from thattool.

(b) If the design has asymmetry with pocket size varying from 50 μm to800 μm (the shorter side of the rectangular pocket or the diameter of adisc), the etch rate of Si in ICP will vary significantly as biggerpocket gets etched faster. Eventually it will take almost double thetheoretical time (400 nm/cycle, each cycle is 17 seconds long) tocompletely release suspended structures from all the pockets. Mostimportantly BOE etching of that thick oxide with a large variety inpocket size is literally impossible to control, resulting inunder-etched or over-etched SiO₂ mask and eventually a total mess afterSi etching with that hard mask. The fabrication yield would be very lowwith this process.

(c) The tool time required for the ICP would be ˜12 hours for releasingall the structures, assuming 1000 μm thick (taking into account for thedifferent pocket sizes) Si and etch rate of 400 nm/cycle. That much deepSi etching would obviously result in a lot of undercut.

To account the above mentioned problems and to ensure higher fabricationyield with zero undercut in the microcantilevers, new process wasdesigned. The details of this new process are described below:

Thinning Down of Bare Si Substrate:

To deal with ˜800 μm thick Si, the samples were first thinned down inSTS® inductively coupled plasma (ICP) using the Bosch recipe to make thethickness about 400 The other recipe can be used just with SF₆ etchcycle with no passivation cycle which would be faster. However,selectivity ratio would be lower with SiO₂ (measured to be 40:1 insteadof 90:1). But this does not affect anything at all as long as thecarrier wafer has enough oxide (in this case the thickness was 9 μm). Tomount the sample cool grease was used carefully on the top side, at thecorners and open area outside 1.4 cm square box. As there will be noresist removal step in this whole process, unfortunately the top surfacewas not protected with any resist coating. Also the resist may get crosslinked for this long duration of Si etching, so if possible the resistcoating on the top surface should be avoided. Another important thingis, if the cool grease is not applied enough, the samples get very hotand metal layers get peeled off from the surface. So this step was donein intervals with 260 cycles runtime with 10 minutes pause. Total 760cycles of the Bosch recipe was run to etch ˜350-400 μm Si with an etchrate of ˜500 nm/cycle (the etch rate is higher as bare Si was etched).The tool time was ˜4 hours.

Oxide Deposition:

As the thinned down sample has become ˜400 μm thick, so a total of 4 μmthick oxide was deposited in UNAXIS PECVD tool in two slots. After 2 μmdeposition (50 nm/min) a clean process was run for 2 hours and the final2 μm was deposited. Though from the selectivity 5 μm thick oxide seemsnecessary, but the photo resist would provide the extra etching cycles.Also, even if the oxide gets etched down but Si still remains unetched,the pattern would be already there, and the Si substrate would only getthinned down which will not harm anything. It is a good practice toprepare carrier wafer which would be the prime Si wafers or any clean Siwafer with at least 8 μm thick oxide. Each wafer should be used once inthe ICP. The tool time was 2 hours and 40 minutes in UNAXIS PECVD and itis same in STS® plasma-enhanced chemical vapor deposition (PECVD) 2. Butthe later has better quality oxide than the former with only drawback isless number of samples can be loaded. If time permits, it is better touse the later tool to deposit oxide following the same procedure.

Photolithography:

The thinned down and oxide deposited sample was patterned with NR 5photoresist. The reason for using NR 5 was its thickness, minimum being8 μm (at 3000 rpm) and maximum being 100 μm (at 500 rpm). The resistacts as a mask not only for etching oxide but also during Si etching.The selectivity was found to be 1:1 with oxide in reactive ion etch (ME)and 40:1 with Si in ICP. So there should about 4 μm resist left afteretching oxide to cushion against etching the first 140-160 μm Si. Thatalso helps in depositing thinner oxide film. However care should betaken to choose the thickness of the resist, as the resist gets thickerafter development the profile does not remain steep and the resist looseits integrity for further processing. The optimized thickness was foundto be 8 μm which gave good results. Up to 15-20 μm thickness would befine with NR 5. Both NR 5 and NR 71 are good etch resist but NR 71offers maximum thickness of 12-14 μm but is less reliable. The lithostep is same as previous, but after the development oxygen plasmacleaning can be run for 1-2 minutes to ensure no resist film isremaining in the pockets. It is not mandatory as the ultimate etchingtime very long which would eventually etch down the thin resistresidues.

Dry Etching of Oxide:

The 4 μm thick oxide was etched down using NR 5 as the mask in two slotswith 2 μm film being etched every time and running a complete cleanprocess for 3 hours in between in Plasma Therm RIE. The etch rate is 50nm/min but overetching was done (assuming 5 μm thickness) to ensurecomplete etching of the oxide from the pocket. A gradient of color canbe seen in open eyes up to 80-90 μm thickness. Then microscope could beused to ensure further etching. As the backside is rough so it becomesharder to justify if few nm film of oxide is remaining. However it willagain not affect due to longer etching of Si. This tool usually makesthe sample contaminated which however did not affect further processing,but it is highly recommended to use VISION® ME for etching oxide. Inthat case, selectivity and etch rate should be measured. It is to benoted that, as the etching was done assuming 5 μm thick oxide, theremaining resist would be 3 which would be good enough to support.Before optimizing the process, two samples were simultaneously processedbut one was used in reactive ion etch (ME) to etch oxide and the otherone was etched with BOE to compare the results. After the etching, thedamages due to BOE was visible but still it was processed further. Thetotal tool time was ˜4 hours.

Deep Si Etching with Bosch Process:

The samples (˜400 μm thick Si substrate) were mounted on carrier waferwith sufficient cool grease. While applying grease with swab on the topsurface, the nearby area surrounding the top pocket (where the GaN wasetched) was avoided as the exposed cool grease (after etching Si) woulddeposit contaminated film and sputtered all over the sample. The Boschrecipe was used and the samples were processed for 1000-1200 cycles inslots of 250 cycles and 10 min pause in between, so that the samples donot get over heated. Over etching does not affect as GaN is barelyetched with SF6 (about 200-300 nm). However, in the new wafer, thecantilever thickness is 1.1 μm after mesa etching. So care should betaken or this can aid in thinning down GaN slowly if different thicknessof cantilever is required. Visual inspection would be enough to ensurecomplete etching and also the samples will be auto diced as per design.The total tool time in STS® ICP was ˜6 hours.

The newly developed process offered the following advantages:

1. Absolutely no undercut, no overhang, and the fabrication yield is100% with releasing about 1000 microcantilevers and suspended structure.

2. Total process time is about 18 hours including tool time andlithography process compare to 30 hours process time with previousprocess.

3. The usual process is absolutely not applicable with more complexdesign such as this which involve dense integration of microcantilevers.

4. No BOE handling at all which not only damages metal stack but alsovery dangerous if exposed to human body.

The epilayer GaN and Si(111) substrate has lattice mismatch and thermalexpansion coefficient difference. Moreover during growth of GaN on Si(111) there is an internal stress distribution due to inhomogeneousoutgrowth of the layer. This causes a residual tensile stress componentin the epilayer. The residual stress in the GaN layer is influenced bygrowth conditions, layer thickness, and layer structures, as well aschoice of substrate. However, during the release of the cantilever thereis a change in stress which pulls the cantilever upwards resulting incurled up structures. The longer microcantilevers have more bendingcompared to the shorter ones.

Example 2

Total resistance of the HFET (externally measured),R_(DS)=R_(int)+2R_(C)+2R_(acc), where R_(acc) is the access regionresistance, R_(c) denotes the source and drain contact resistances.R_(int) is the drain-source resistance of the intrinsic transistor,where the gauge factor, GF, can be derived as (derivation is given inthe last section),

$\begin{matrix}{{GF} = {\frac{\frac{\Delta\; R_{DS}}{R_{DS}}}{ɛ} \approx {- {\frac{1}{ɛ}\left\lbrack {{\Delta\;{\mu_{int}/\mu_{int}}} + {\Delta\;{n_{s,{int}}/n_{s,{int}}}}} \right\rbrack}}}} & \left( {{Equation}\mspace{14mu} 2.1} \right)\end{matrix}$Here, μ_(int) and n_(s,int) are the mobility and carrier concentrationfor the intrinsic device, and c is the average strain in the channel. Itis obvious from Eqn. 2.1 that the GF depends on both changes in carrierconcentration and mobility, which are strongly correlated at gate biasesclose to pinch-off (i.e. lower carrier concentration). Clearly, thisresults in a higher GF in a gated piezoresistor, where the gate voltagecan be used to tune the carrier concentration to a desired (low) levelwhere the mobility would change significantly due to change in carrierconcentration, in addition to higher fractional change in the carrierconcentration itself (caused by external strain). For a Si piezoresistor(i.e. p-type Si) the carrier concentration does not depend on externalstrain, so the additional benefit of mobility change, caused by changein carrier concentration as noted above, is absent.

In a simple AlGaN/GaN piezoresistor, without the possibility of gatemodulation, the carrier concentration does change with strain but theadditional advantage of mobility change is uncertain.

Step Bending

To determine the step bending response, the microcantilever was bentdown by 1 μm and released, as VGS was systematically varied. Downwardbending causes larger tensile strain in the AlGaN layer, which in turn,generates more positive piezoelectric charge at the AlGaN/GaN interface,drawing excess compensating electrons (Δn_(s)), and thereby reducingRDS. When the cantilever is released, excess tensile strain is reduced,and RDS returns to its initial value. With more negative V_(GS) applied,n_(s) reduces, which increases the ratio Δn_(s)/n_(s) and maximizesΔR_(DS)/R_(DS) and hence the GF. The step bending response of thisdevice, for V_(GS)=0 and −3.1 V was found to be R_(DS)=1 kΩ andΔR_(DS)=7Ω, whereas V_(GS)=−3.1 V yielded R_(DS)=2.16 MΩ and ΔR_(DS)=300kΩ. Thus, ΔR_(DS)/R_(DS) increased more by 2 orders as V_(GS) approachedthe shutdown voltage of the HFET of −3.2 V. The computed sensitivity(=ΔR_(DS)/R_(DS)) increases monotonically from V_(GS)=0, and reaches amaximum value of 13.8% at V_(GS)=−3.1 V. The average strain on the HFETwas estimated as 4.3×10⁻⁵ from the finite element COMSOL simulation. Amaximum GF=3200 is calculated at V_(GS)=−3.1 V, which decreasesmonotonically as the V_(GS) increases to more positive values. It isnoteworthy that the maximum GF calculated here is 35 times higher thanthe optimized Si based piezoresistive devices (GF=95), and comparable tothat of SWCNT (GF=2900). The sensitivity of this device did not varysignificantly with Vbs. However, with more negative V_(GS), especiallynear shutdown, the HFET was operated in the saturation region to enableI_(DS) to dominate over the gate leakage current. Although significanttransients were observed for more negative V_(GS) when the cantileverwas bent down and released, we only considered the steady state valuesof R_(DS) for calculating GF. If the maximum transient value of R_(DS)is used we would get a much higher GF of about 10,000. It is possiblethat if the transients are minimized through proper device passivationthen even higher GF can be achieved.

Dynamic Bending

For dynamic response, an oscillating piezochip was contacted to the DIP,which generated a surface wave that propagated to the cantilever toinitiate oscillation. The oscillation of the microcantilever wastransduced by the HFET (biased with constant IDS=10 μA and V_(GS)=−2.3V), where the R_(DS) changed periodically, resulting in a periodicchange in the drain-source voltage, ΔV_(DS), which was measured by thelock-in amplifier. Laser vibrometer measurements very closely matchedthe HFET measurements, which yielded a resonant frequency of 43.94 kHzwith a quality factor of 230. The voltage responsivity (VR) which is amore important parameter than GF for dynamic bending, was determined bytaking the ratio of ΔV_(DS) and the oscillation amplitude consideringthe difference of the on-resonance peak and off-resonance base.Comparing the two measurements, we find that a change in oscillationamplitude of 7.9 nm (from vibrometer) corresponded to ΔV_(DS) (rms)=7.5μV (from HFET). Thus, the VR can be calculated as 0.95 μV/nm. Similarlyas in step bending case, more negative V_(GS) resulted in increasedΔR_(DS) which enhanced the responsivity, since ΔV_(DS)=I_(DS)×ΔR_(DS).With decrease in V_(GS), VR increases monotonically, reaching a value of40 μV/nm with the same piezo excitation, at V_(GS)=−2.7 V and IDS=10 μA.The power dissipation across the HFET was calculated using P_(DS)=I_(DS)²×R_(DS) for different V_(GS) using I_(DS)=10 μA and R_(DS) values. Itwas found that P_(DS) increases monotonically from 0.51 μW to 2.4 μW, asV_(GS) becomes more negative, changing from ˜2.3 V to −2.7 V. Thepiezoresistive response of the HFET is limited mainly by the Johnsonnoise at high frequency which is given by, S_(J)=√(4 k_(B)TR_(DS)B),where k_(B)T=26 meV at room temperature and B is the measurementbandwidth. With B=10 Hz, the calculated Johnson noises were 28.84 nV and139.42 nV for V_(GS)=−2.3 V and −2.7 V, respectively, while thecorresponding signal-to-noise ratios (SNR=20 log₁₀ (VR/S_(J))) are 30.35dB and 49.15 dB, for 1 nm oscillation amplitude. However changing thebias current from 10 μA to 100 μA, sharply increased the SNR from 30.35dB to 73.7 dB. Clearly, there is a trade-off between three criticalparameters of a HFET deflection transducer, namely, power dissipation,responsivity and SNR. For example, for V_(GS)=−2.3 V, we obtained thehighest responsivity of 140 μV/nm with an SNR of 73.7 dB, however thiswas achieved at the cost of higher power dissipation of 51 μW. We wouldlike to mention here that this device and other similar devices haveshown excellent repeatable and reproducible performances as mentionedabove when tested several times in one year time period.

Our gated piezoresistor offers the advantage of utilizing the samedevice to cater to various application needs (i.e. requirement of lowpower consumption, high sensitivity, high SNR, or DC to ultrasonicfrequency operations), simply by biasing the transistor. Theexperimental results presented here provide the necessary insights intothe operation of HFET embedded micro/nano cantilever.

Rectangular Microcantilever Device Performance

One of the newly fabricated microcantilevers (length is 150 width 50 μmand thickness is 1 μm) was tested. Impressive and better performanceswere observed. The transmission line measurement (TLM) results on TLMpads which yielded contact resistance of 13.39Ω and sheet resistance of478.1Ω/□. The device also showed excellent gate control and very highcurrent with low leakage as expected from usual AlGaN/GaN HFET. Similaras described earlier, static bending test was performed and the devicepresented 140% change in HFET channel resistance for 10 μm bending.

Triangular Microcantilever Device Performance

One of the newly fabricated triangular microcantilevers (V shaped,height 250 μm, width and thickness 1 μm) was also studied with bothstatic bending and dynamic bending characterizations. The V shapedcantilevers have two arms and so two HEFT with similar or differentorientations considering current conduction, were integrated. Howeverthe chosen one was with two similar HFETs. Two of the HFETs were eitherused together or separately to transduce the mechanical deflection ofthe V shaped Microcantilever. In that case the biasing parameters werekept same for both HFETs when acted as a single HFET. The channelresistances (R_(DS)) were measured to be 850Ω and 1.2 kΩ for thedevices. As described earlier, the higher the resistance, the higher thesensitivity (or gauge factor), so it is presumed that HFET with thehigher resistance will present higher sensitivity. However, as we haveseparate gate controls we can tune the gate bias to match theresistances to obtain equal sensitivity. The mechanical arms aresymmetrical, so if the external stress is applied in the middle of thetip equal strain would be distributed at the two bases yielding equalpiezoresistive changes. However in this experiment we have kept thedrain-source and gate bias same and the cantilever was bent 1 μmdownward and released. The bending results were analyzed, when the twoHFET transduced separately, and when they were connected togetherexternally (with jumper cables shorting two sources and drains). Thesensitivities were measured to be 0.44% (for one HFET), 0.57% (for theother HFET), and 0.48% (both) per 1 μm.

The dynamic responses were also measured with this cantilever asdescribed earlier (the measurement). The cantilever was oscillated withPiezo actuation. The resonance frequency was found 47.871 kHz and thequality factor was 371. Fortunately there was a dust particle on thecantilever which allowed us to measure the mass loading on themicrocantilever and the corresponding frequency shift, which thefrequency downshift of the resonance frequency of the cantilever wasfound to be by 721 Hz when a dust was on the cantilever. The biasoptimization was not performed on this particular cantilever. But thebiasing parameters were: constant I_(DS)=10 μA, V_(GS)=−3.0 V.

Example 3

The ultrahigh deflection sensitivity achieved using an AlGaN/GaNheterojunction FET (HFET) embedded piezotransistive GaN microcantileverwas demonstrated, which resulted in successful transduction offemtometer level displacement at the resonance frequency of thecantilever. The capability of measuring these extremely smalldisplacements, verified independently through laser vibrometry studies,has enabled detection of nanogram level explosives with high specificityusing novel surface based photoacoustic technique.

Piezotransistive microcantilevers were fabricated using III-Nitrideepitaxial layers grown on Si (111) substrate. The overall layerstructure was i-GaN (2 nm)/AlGaN (17.5 nm, 26% Al)/i-GaN (1μm)/Transition layer (1.1 μm)/Si (111) substrate (500 μm), wherein“i-type semiconductor” indicates “undoped semiconductor” or “intrinsicsemiconductor”. The HFET was fabricated with initial 200 nm mesa etchingfollowed by GaN cantilever pattern etched down using BCl₃/Cl₂ basedinductively coupled plasma etch process. Ohmic contacts were formed withTi (20 nm)/Al (100 nm)/Ti (45 nm)/Au (55 nm) metal stack deposition andrapid thermal annealing. Schottky gate contact was then formed with Ni(25 nm)/Au (375 nm) deposition. Finally, through wafer Si etch wasperformed by “Bosch process” to release of the microcantilevers.

Detection of Nanoscale Static Deflection

The fabricated microcantilevers had dimensions of 250×50×2 μm³, with theembedded HFET's channel dimension being 17×29×6 μm³. Each chip had 4similar microcantilevers, which were wire bonded to a 28 pindual-in-line package (DIP) chip carrier. Apart from the conventionalsource, drain, and gate contacts of the HFET, there was an additionalcontact for electrostatic actuation of the microcantilever, which wasnot used in this study. Typical IDS-VDS and IDS-VGS characteristics ofthe HFET, exhibiting good gate control. Utilizing a negative gate biasthe piezoresistive effect was translated into a piezotransistive effectwhere the 2DEG carrier concentration (ns) 140 was reduced, thusincreasing the Δns/ns ratio (Δns is the change in 2DEG density due tostrain caused by deflection of the cantilever).

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood the aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

The invention claimed is:
 1. A method of fabricating a GaN cantileverfor a photoacoustic sensor from a wafer, wherein the wafer comprises asubstrate, a buffer layer on the substrate, and a first undoped GaNlayer on the buffer layer, the method comprising: forming a mesacomprising an AlGaN layer over a portion of the wafer; defining acantilever outline in the first undoped GaN layer, wherein the mesa ispositioned near or on the base of the cantilever outline; depositing atleast two contacts onto the mesa; and through backside etching, removingthe wafer in an area of the cantilever outline to release thecantilever, wherein the cantilever has integrated near or at the base ofthe cantilever a heterojunction field-effect transistor (HFET)deflection transducer, wherein the HFET deflection transducer has across-section comprising the following layers arranged in consecutivehorizontal order: the buffer layer directly on the substrate; the firstundoped GaN layer directly on the buffer layer; an AlN layer directly onthe first undoped GaN layer; the AlGaN layer directly on the AlN layer;and an additional undoped GaN layer directly on the AlGaN layer.
 2. Themethod of claim 1, further comprising: forming, through etching,microchannels and a reservoir within the first undoped GaN layer nearthe mesa.
 3. The method of claim 2, further comprising: coating themicrochannels and the reservoir with a poly(p-xylylene) polymer.
 4. Themethod of claim 2, further comprising: encapsulating the microchannelsand the reservoir by securing a sapphire window thereto.
 5. The methodof claim 2, further comprising: encapsulating the cantilever with asapphire window such that the cantilever is positioned within anenclosed space.
 6. The method of claim 5, further comprising: forming avacuum within the enclosed space so as to maintain a vacuum around thecantilever.
 7. The method of claim 1, wherein the substrate comprisessilicon.
 8. The method of claim 1, wherein the cantilever is fabricatedwith a length of from 50 μm to 300 μm, a width of from 25 μm to 100 μm,and a thickness of from 0.5 μm to 1.5 μm.
 9. The method of claim 1,wherein the substrate comprises silicon, and wherein the buffer layerhas a thickness from 0.5 μm to 2.5 μm, and further wherein the firstundoped GaN layer has a thickness from 500 nm to 2 μm.
 10. The method ofclaim 1, wherein the cantilever for a photoacoustic sensor is fabricatedto have a gauge factor greater than 4500.